Noise-equivalent temperature (NET) is a measure of the sensitivity of a detector of thermal radiation in the
infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
,
terahertz or
microwave
Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
portions of the
electromagnetic spectrum
The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high ...
. It is the amount of incident signal
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
that would be needed to match the internal noise of the detector such that the
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...
is equal to one. Often the spectrum of the NET is reported as a temperature ''per root bandwidth''. A detector that measures power is often interested in the analogous
noise-equivalent power Noise-equivalent power (NEP) is a measure of the sensitivity of a photodetector or detector system. It is defined as the signal power that gives a signal-to-noise ratio of one in a one hertz output bandwidth. An output bandwidth of one hertz is equ ...
(NEP). If a relation between intensity and temperature is well defined over the
passband
A passband is the range of frequency, frequencies or wavelengths that can pass through a Filter (signal processing), filter. For example, a radio receiver contains a bandpass filter to select the frequency of the desired radio signal out of all t ...
, as in the case of a
blackbody
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The radiation emitted by a black body in thermal equilibrium with its environment is ...
, then the NET simply scales with the NEP.
If a detector is limited by either
shot noise
Shot noise or Poisson noise is a type of noise which can be modeled by a Poisson process.
In electronics shot noise originates from the discrete nature of electric charge. Shot noise also occurs in photon counting in optical devices, where s ...
or
Johnson noise then the NET can be decreased by using an increased integration time. The NET of
flicker noise
Flicker noise is a type of electronic noise with a 1/''f'' power spectral density. It is therefore often referred to as 1/''f'' noise or pink noise, though these terms have wider definitions. It occurs in almost all electronic devices and can show ...
limited detectors can not be reduced by increased integration time.
Typically uncooled
bolometric detectors have NET figures of 30-200
mK. Cooled photon detecting infrared detectors using materials such as
HgCdTe
Hg1−''x''Cd''x''Te or mercury cadmium telluride (also cadmium mercury telluride, MCT, MerCad Telluride, MerCadTel, MerCaT or CMT) is a chemical compound of cadmium telluride (CdTe) and mercury telluride (HgTe) with a tunable bandgap spanning th ...
(
LWIR
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...
or
MWIR
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...
) or
InSb
Indium antimonide (InSb) is a crystalline compound made from the elements indium (In) and antimony (Sb). It is a narrow- gap semiconductor material from the III- V group used in infrared detectors, including thermal imaging cameras, FLIR syst ...
(MWIR) can approach a NET figure of 10 mK. In the microwave radiation region NET values are typically several hundred millikelvins to several kelvins.
For a particular mean signal temperature there is a fundamental limit to NET given by the natural thermodynamic fluctuations of the photon flux from the source under investigation.
See also
*
Noise-equivalent power Noise-equivalent power (NEP) is a measure of the sensitivity of a photodetector or detector system. It is defined as the signal power that gives a signal-to-noise ratio of one in a one hertz output bandwidth. An output bandwidth of one hertz is equ ...
*
Specific detectivity
Specific detectivity, or ''D*'', for a photodetector is a figure of merit used to characterize performance, equal to the reciprocal of noise-equivalent power (NEP), normalized per square root of the sensor's area and frequency bandwidth (reciproc ...
*
Minimum resolvable temperature difference
Minimum resolvable temperature difference (MRTD) is a measure for assessing the performance of infrared cameras, and is inversely proportional to the modulation transfer function.
Typically, an operator is asked to assess the minimum temperature ...
External links
Expanding the Vision of Sensor Materials (1995)National Materials Advisory Board (NMAB) {{webarchive , url=https://web.archive.org/web/20120812224324/http://www.nap.edu/openbook.php?isbn=0309051754&page=122 , date=August 12, 2012 , title=Expanding the Vision of Sensor Materials (1995)
Infrared imaging
Equivalent units